ENGR2000_phasetransformations

ENGR2000_phasetransformations - Introduction to Engineering...

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Unformatted text preview: Introduction to Engineering Materials ENGR2000 Chapter 10: Phase Transformations in Metals Dr. Priya Thamburaj Goeser 10.2 Basic Concepts Phase transformations involve alteration of the microstructure Change in number of phases Change in composition of phases Diffusion dependent transformations Diffusionless transformations Terminology for microstructures in Fe-C alloys Steels Plain carbon steels Steels in which C is the primary alloying element Alloy steels Contain significant concentrations of alloying elements such as Cr, Ni, Mo, W, etc. - ferrite - low in C and for most cases, can be considered pure Fe 10.5 Isothermal Transformation Diagrams IT diagrams Represent time & temperature dependence of transformations Eutectoid reaction ( ) ( ) ( ) C wt C Fe C wt C wt .% 7 . 6 .% 022 . .% 76 . : reaction Eutectoid 3 + Iron-Iron carbide phase diagram Isothermal Transformation diagram for the eutectoid alloy (0.76 wt % C) Points to note on using IT diagrams Diagrams are valid only for specific alloy compositions. Diagrams are accurate only for transformations in which the temperature of the alloy is held constant throughout the duration of the reaction. Pearlite - a two-phase mixture with layers of alternating ferrite & cementite Cooling 0.76 wt. % C alloy to just below the eutectoid temperaturesingle phase austenite transforms to pearlite Coarse pearlite and fine pearlite Coarse pearlite At temperatures just below the eutectoid, relatively thick layers of both the -ferrite and cementite phases are produced. Fine pearlite With decreasing temperature ~ 540 C, the carbon diffusion rate decreases and the layers become thinner. Microstructure of pearlite...
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This note was uploaded on 10/24/2009 for the course ENGR 2000 taught by Professor Goeser during the Fall '08 term at Armstrong State University.

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ENGR2000_phasetransformations - Introduction to Engineering...

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